The present invention relates generally to optical systems and more particularly to optical computing, information and communications systems and logic elements for use therein which utilize the principle of cross-phase modulation (XPM).
When the intense ultrashort light pulse propagates through a non-linear material, it temporally distorts the atomic and molecular configuration of the material. This distortion of the non-linear material instantaneously results in a change in the refractive index of the material. This change in the index of refraction is directly proportional to the intensity of the propagating intense light pulse. The change in the refractive index of the non-linear material, in turn, causes a phase change in the propagating intense light pulse. The phase change causes a frequency sweep within the pulse envelope, typically resulting in a blue shift at the tail end of the pulse and a red shift at the front of the pulse. Typically, the effect is a spectral broadening of the pulse resulting in the generation of a supercontinuum. This spectral effect on the propagating intense light pulse is typically referred to as a self-phase modulation effect.
In addition to experiencing self-phase modulation, an intense light pulse propagating through a non-linear material will typically undergo self-focusing, that is, a narrowing of the cross-sectional diameter of the pulse. Self-focusing occurs because, typically, the intensity of a pulse of light is greatest at its center and weakest at its outer edges. Since n is directly proportional to the intensity of the pulse, the center of the pulse causes a greater change in refractive index of the non-linear material than the outer edges of the pulse. Consequently, the center of the pulse travels slower than its outer edges, causing the outer edges to bend in towards the center of the pulse. This effect causes the beam to focus.
In addition to experiencing self-phase modulation and self-focusing, an intense light pulse propagating through a non-linear material may also be used to induce the phase modulation of and/or the focusing of a co-propagating weak light pulse. These phenomena are typically referred to as cross-phase modulation and induced focusing, respectively.
Cross-phase modulation may result in either frequency shifting (i.e., blue shifting or red shifting) or spectral broadening (i.e., supercontinuum generation), the particular effect depending on the relative times at which the weak pulse and the intense pulse propagate through non-linear material. For example, if the intense pulse has a greater wavelength than the weak pulse, the intense pulse will travel faster through the non-linear material. Therefore, if the intense and weak pulses are sent propagating into the non-linear material at the same time, the weak pulse will be exposed predominately to the change in refractive index caused by the tail end of the intense pulse. (This is referred to commonly as tail walk-off). The result of tail walk-off is a blue shift of the weak pulse. Analagously, if the weak pulse is sent propagating into the non-linear material ahead of the intense pulse, the weak pulse will feel the effects of the refractive index change due to the front end of the intense pulse (front walk-off). The result of front walk-off is a shift of the weak pulse to the red. Finally, if the weak and intense pulses are sent propagating into the non-linear material so that the weak pulse is subjected to the changes in the refractive index caused by both the tail end and the front end of the intense pulse (e.g. symmetric walk-off or no walk-off), the weak pulse broadens spectrally to both the red and the blue.
Spectral changes arising from cross-phase modulation may lead to changes in the temportal profile of the weak pulse when it propagates into a dispersive medium (i.e. an optical fiber) or a dispersive optical component (i.e. a grating or a prism). For example, if cross-phase modulation results in the spectral broadening of the weak pulse, a further propagation of the weak pulse through a grating pair may slow down its re-shifted frequencies (generated by XPM at the pulse front) with respect to its blues shifted frequencies (generated by XPM at the pulse back), and consequently reduces the pulse duration of the weak pulse.
Cross-phase modulation may also be used to change the spatial distribution of copropagating weak pulses. This effect occurs when the intense pulse generates a spatially-dependent non-linear refractive index. For example, a pump pulse with a Gaussian spatial distribution of its intensity generates a higher refractive index on the propagation axis of the weak pulse. As a consequence, the outer edges of the weak pulse bend in towards the center of the pulse, and the weak pulse focuses.
As a term of art, cross-phase modulation is frequently used generically to refer to both cross-phase modulation and induced focusing.
Non-linear materials are very well known in the art. Examples of non linear materials are BK-7 glass, CdSe, liquid Cs.sub.2, NaCl crystal, doped glasses, semiconductor bulk and quantum structures, microcrystalline semiconductor particles in glasses polydicetylene organic polymer and optical fibers.